1. Field of the Invention
The invention relates to a nuclear core catching apparatus adapted to be disposed below the core of a nuclear reactor for containing the components of a nuclear reactor core during a core-melt-down accident.
2. Description of the Prior Art
If in a loss-of-coolant accident during the operation of a nuclear reactor the emergency core cooling system fails, parts of the core may melt as a result of the heat produced during the rapid decay of the fission products formed by fission of the nuclear fuel atoms during operation of the nuclear reactor.
Following a loss-of-coolant accident, a nuclear reactor is rapidly shut down by insertion of all control rods into the reactor core. However, even if the reactor is properly shut down and heat production by nuclear fission has ended, the decay of the fission products in the nuclear reactor core may still generate sufficient heat to melt the core if the core emergency cooling system fails to cool the core after reactor shutdown.
How serious such a core melt-down becomes, that is particularly, how long the melt remains liquid thereby forming a threat of destruction to the reactor vessel or even the reactor containment, depends to a great extent on the degree of removal of the fission products from the nuclear fuel. Removal of the fission products from the melt naturally reduces the heat generation in the melt. However, knowledge of the size of any heat sources remaining in the melt is considered to be necessary for the determination of apparatus designed for the protection of the containment, such as core catchers. Each core catcher must be designed for accommodation of all the heat that may be generated in a core after reactor shutdown. In accordance with the present state of the art, it is believed that about 65 .+-. 15% of all radioactive heat sources remain in the melt after core melt-down.
It is possible that, as a result of a core melt-down, the reactor vessel bottom is melted or even the concrete foundation of the containment is destroyed so that fission products are released. This, of course, forms a great threat to the environment.
It is furthermore possible, particularly in water-cooled reactors, that exotherm chemical reactions take place between the coolant and the fuel rod cladding or other structural materials which usually result in the formation of hydrogen. However, such reactions could also occur in gas cooled reactors if, for example, a break occurs in the steam generator resulting in a water break-in into the core melt.
To avoid the formation of a critical mass after a core-melt-down accident, a core catcher has been proposed (German DOS No. 2,259,071) which consists of a number of superimposed pan-type containers with a central body having a conical head at its upper end. Below its conical head, the central body has two superposed grid structures which are provided with vanes and which distribute the melt and retain fuel particles of various sizes. The liquid melt passes through the grid structures and is retained in the pan-type containers disposed below the grid structures.
Such core catching containers have the purpose of containing the melt within the reactor containment. The decay heat is carried away partially through the walls of the container and partially from the surface of the melt in the container.
If removal of the heat produced during a core-melt-down accident fails it is easily possible that structural portions of the nuclear reactor or even parts of the containment are melted. The core catching devices known so far will then no longer afford sufficient protection.
The possibility of injecting water into the containment for the purpose of cooling the core can be considered only for water -- or gas-cooled reactors and only if the problems associated with the formation of hydrogen are solved. However, it is not possible to inject water into the containment for the cooling of cores of liquid metal cooled reactors.
It has been proposed to provide core catching pans consisting of uranium dioxide or thorium dioxide. The fabrication of such pans, however, would be extremely difficult if at all possible because of the adverse mechanical properties of these ceramic materials. In addition, the costs of manufacturing core catching pans of these materials would be very high.